Drying is one of the oldest methods of preserving foodstuffs. The use of the sun and fire as drying agents is even mentioned in the bible; during the first and second world wars, many types of food were dried in an attempt to develop light, nutritious, stable and easily preparable foods for use by the military.
Later, people turned ever more to the production of dehydrated foods to feed groups and individuals facing long periods of isolation such as astronauts, explorers and mountaineers, or rather foods for preparation and consumption under extreme conditions.
The development of technologies and the patenting and manufacture of machinery adapted to providing perfectly dehydrated foodstuffs without any loss of aroma and flavour has lead to the ever increasing spread of dried foodstuffs, which are now finding daily use. Thus, besides the innumerable frozen, precooked and vacuum packed products, the evolution of dietary habits has also lead to the development of a wide range of dried and freeze-dried products.
Drying preserves foodstuffs by lowering the quantity of water or humidity in the foodstuff's matrix. This is essential for impeding microbial growth and the chemical degradation reactions which normally occur in fresh foodstuffs.
Maximising the reduction in water content delays or halts microbial development, since microorganisms cannot survive below a certain humidity threshold.
The drying conditions and the physico-chemical changes which occur during the process have an influence on product quality; more specifically, the drying method has a significant impact on colour, consistency, density, the porosity and absorption characteristics of the food material which will then be rehydrated. The same foodstuff can give rise to a completely different product depending on the type of dehydration it has undergone.
Subsequent rehydration depends on the level of cellular and structural destruction that has occurred during dehydration.
Foodstuffs may be dehydrated by exploiting a number of techniques.
Drying using the sun's rays is the oldest and most traditional method. Said technique requires constant exposure to direct sunlight during the day, in a situation with relative humidity of less than 20%. Foodstuffs dried this way may require 3-4 days to dry completely and are exposed to dirt, insects and rodents etc.
Conventionally, drying uses heat propagation. The most common examples include: The static heat propagation system wherein the foodstuffs are arranged in 2-3 cm layers over steel basins located in hot air circulating cabinets, at more or less 40-50° C.; the dynamic tunnel system where the trays containing the foodstuffs to be dried are placed on a conveyor belt which passes through a tunnel in the opposite direction to a current of temperature and humidity controlled air; the rotating cylinder dynamic system wherein the foodstuff materials are introduced into one end of a long cylinder in the opposite direction to a current of hot air, which results in the continuous remixing of the materials. Finally, there is fluid bed drying. Said method uses “fluidisation” technology wherein a layer of foodstuff material is positioned on a grid located at the base of a truncated conical container and a current of air is passed through from below at increasing speed.
In the conduction drying method, heat transfer occurs through a solid wall, without any movement of material.
Drying by irradiation, for example, is carried out by using infrared rays, by placing the material on conveyor belts which pass through tunnels in which suitable lamps are located.
These traditional methods, which can be applied to suitably cut or chopped raw or precooked foodstuffs have been more recently supplemented with more sophisticated methods.
This is the case for the drying of solutions or purees of foodstuffs. Indeed, prior to drying, foodstuffs frequently undergo specific processing transforming them into purees and solutions; this is the case for example with mashed potato or concentrated tomato juice. Such solutions are then transformed into powdered, flaked or granular products. This is realised thanks to drum or spray driers. In drum driers (drum drying) the wet food falls between one or two internally heated cylinders which dry the product. The latter is then “grated” by means of scouring blades which remove it from the surfaces of the drum. With spray dryers (spray drying), the liquid foodstuff is nebulised and the mist introduced from above into a hot air counter-current originating from below. The product dries and is separated from the air.
The powders thus produced may then be compacted and transformed into granules, using for example a roller compactor, a device fitted with rotating cylinders which produces granules.
Another drying process, even more widely used for producing dried foodstuffs is lyophilisation, which is achieved by means of rapid freezing followed by sublimation, and subsequent drying of the remaining food-bound water (freeze-drying). The low temperatures used in lyophilisation impede any chemical changes which might occur due to the numerous thermolabile components present in the food. Furthermore, lyophilisation guarantees the permanent dispersion of the food material, which dries without undergoing concentration, and forms a porous solid which dissolves rapidly and effectively when necessary.
Thus a solid, porous, friable, hygroscopic, very soluble mass is obtained, which occupies the same volume as the original frozen mass.
Somogyi and Luh, Vegetable dehydration. Commercial fruit processing; Woodruff, J. G., Luh, B. S., Eds.; Avi Publishing Co.: Westport, Conn., 1986, 435, report a study of the effects of the drying method on the quality of green asparagus where they refer that the lyophilised asparagus reconstituted in hot water shows much greater tenderness and solubility than the hot air dried product. Numerous other studies on the topic report similar findings. Other studies show that the low temperatures used in lyophilisation allow greater retention of flavours and volatile components with respect to other drying methods.
Traditional drying techniques are largely used in the food industry. In particular, on the market, there are a number of products, for example various flavours of instant sauces and gravies, which are mixtures of ingredients dried using traditional methods.
Such products have certain advantages. Normally, they are mixtures of flakes or powders of various types of vegetables, tomatoes, mushrooms, cheeses etc. obtained using traditional drying techniques and, therefore, very coriaceous and poorly soluble in water. This means there is a need to cook such products for relatively long amounts of time before being able to consume them (six to ten minutes). Furthermore, in order to be able to obtain homogeneous and creamy mixtures, after cooking, the known products contain binding additives such as for example flour and starch, or synthetic emulsifiers such as for example polyglycerin behenate. Despite the presence of emulsifiers, during cooking, the known products give rise to the formation of clumps, preventing the homogeneous mixing and uniform cooking of the product.
Another disadvantage relates to the aesthetic appearance of known dried products, which are normally characterised by pale and bland colours. Furthermore, the consistency of the solid mixture is not homogeneous since powdered ingredients tend to adhere to the inner surfaces of the packaging due to electrostatic phenomena. This is rather unappealing for the consumer, thus known instant sauces are normally packaged in non-transparent packs.
Also, in the known instant sauces, a further disadvantage relates to the presence of other chemical additives, such as for example: preservatives, powdered animal or vegetable fats, flavour enhancers, for example sodium glutamate, and sugars such as dextrose and sucrose. The added sugars in particular make such products unsuitable for diabetics.